The present invention relates in general to lithography and more particularly to systems for preparing lithographic printing plates using digitally controlled laser output. More specifically, this invention relates to methods for preparing a lithographic printing plate especially suitable for directly imaging and utilizing with a wet lithographic printing press. The present invention also pertains to wet lithographic printing plates prepared according to such methods.
Traditional techniques for introducing a printed image onto a recording material include letterpress printing and offset lithography. Both of these printing methods require a plate. To transfer ink in the pattern of the image, the plate is usually loaded onto a plate cylinder of a rotary press for efficiency. In letterpress printing, the image pattern is represented on the plate in the form of raised areas that accept ink and transfer it onto the recording medium by impression. The term xe2x80x9clithographic,xe2x80x9d as used herein, is meant to include various terms used synonymously, such as offset, offset lithographic, planographic, and others. By the term xe2x80x9cwet lithographic,xe2x80x9d as used herein, is meant the type of lithographic printing plate where the printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth, and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced. In a dry lithographic printing system that does not utilize water, the plate is simply inked and the image transferred directly onto a recording material or transferred onto a blanket and then to the recording material.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the aluminum for such use, it is typically subject to both a graining process and a subsequent anodizing process. The graining process serves to improve the adhesion of the image to the plate and to enhance the water-receptive characteristics of the background areas of the printing plate. The graining and anodizing affect both the performance and the durability of the printing plate. Both mechanical and electrolytic graining processes are well known and widely used in the manufacture of lithographic printing plates. Processes for anodizing aluminum to form an anodic oxide coating and then hydrophilizing the anodized surface by techniques such as silication are also well known in the art, and need not be further described herein. The aluminum support is thus characterized by having a porous, wear-resistant hydrophilic surface, which specifically adapts it for use in lithographic printing, particularly where long press runs are required.
The plates for a lithographic press are usually produced photographically. The aluminum substrate described above is typically coated with a wide variety of photo-sensitive materials suitable for forming images for use in the lithographic printing process. Lithographic printing plates of this type are usually developed with an aqueous alkaline developing solution, which often additionally comprises a substantial quantity of an organic solvent.
To prepare a wet plate using a typical negative-working subtractive process, the original document is photographed to produce a photographic negative. This negative is placed on an aluminum plate having a water-receptive oxide surface coated with a photopolymer. Upon exposure to light or other radiation through the negative, the areas of the coating that received radiation (corresponding to the dark or printed areas of the original) cure to a durable oleophilic state. The plate is then subjected to a developing process that removes the uncured areas of the coating (i.e., those which did not receive radiation, corresponding to the non-image or background areas of the original), thereby exposing the hydrophilic surface of the aluminum plate.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
As is evident from the above description, photographic platemaking processes tend to be time consuming and require facilities and equipment adequate to support the necessary chemistry. Efforts have been made for many years to manufacture a printing plate, which does not require aqueous alkaline or solvent development or which only uses water for development. In addition, practitioners have developed a number of electronic alternatives to plate imaging, some of which can be utilized on-press. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. Such imaging devices include sources of electromagnetic radiation, produced by one or more laser or non-laser sources, that create physical and/or chemical changes on plate blanks; ink jet equipment that directly deposits ink-repellent or ink-accepting spots on plate blanks; and spark-discharge equipment, in which an electrode in contact with or spaced closely to a plate blank produces electrical sparks to physically alter the topology of the plate blank, thereby producing xe2x80x9cdotsxe2x80x9d which collectively form a desired image as for example, described in U.S. Pat. No. 4,911,075. Because of the ready availability of laser equipment and its amenability to digital control, significant effort has been devoted to the development of laser-based imaging systems.
In one such system, argon-ion, frequency-doubled Nd-YAG, and other infrared lasers are used to expose photosensitive blanks for traditional chemical processing, as for example described in U.S. Pat. Nos. 3,506,779; 4,020,762; 4,868,092; 5,153,236; 5,372,915; and 5,629,354. In an alternative to this approach, a laser has been employed to selectively remove, in an imagewise pattern, an opaque coating that overlies a photosensitive plate blank. The plate is then exposed to a source of radiation, with the unremoved material acting as a mask that prevents radiation from reaching underlying portions of the plate, as for example described in U.S. Pat No. 4,132,168. However, the need for high writing speeds, coupled with the constraint of the low-powered lasers favored by industry, has resulted in a requirement for printing plates that have a very high photosensitivity. Unfortunately, high photosensitivity almost always reduces the shelf life of these plates.
Another approach to laser imaging uses thermal-transfer materials, as for example described in U.S. Pat. Nos. 3,945,318; 3,962,513; 3,964,389; 4,395,946; and 5,395,729. With these systems, a polymer sheet transparent to the radiation emitted by the laser is coated with a transferable material. The transfer side of this construction is brought into contact with an acceptor sheet, and the transfer material is selectively irradiated through the transparent layer. Irradiation causes the transfer material to adhere preferentially to the acceptor sheet. The transfer and acceptor materials exhibit different affinities for fountain solution and/or ink, so that removal of the transparent polymer sheet with the unirradiated transfer material still on it leaves a suitably imaged, finished plate. Typically, the transfer material is oleophilic, and the acceptor material is hydrophilic. Plates produced with transfer type systems tend to exhibit short useful lifetimes due to the limited amount of material that can effectively be transferred: Airborne dirt can create an image quality problem depending on the particular construction. In addition, because the transfer process involves melting and resolidification of material, image quality further tends to be visibly poorer than that obtainable with other methods.
Other patents describe lithographic printing plates comprising a support and a hydrophilic imaging layer which, upon imagewise laser exposure, becomes oleophilic in the exposed areas while remaining hydrophilic in the unexposed areas, as for example disclosed in U.S. Pat. Nos. 3,793,033, 4,034,183; 4,081,572; and 4,693,958. However, these types of lithographic printing plates suffer from the lack of a sufficient degree of discrimination between oleophilic image areas and hydrophilic non-image areas, with the result that image quality on printing is poor.
Early examples utilizing lasers used the laser to etch away material from a plate blank to form an intaglio or letterpress pattern, as for example described in U.S. Pat. Nos. 3,506,779 and 4,347,785. This approach was later extended to production of lithographic plates, e.g., by removal of a hydrophilic surface to reveal an oleophobic underlayer, as for example described in U.S. Pat. No. 4,054,094. These early systems generally required high-power lasers, which are expensive and slow.
Other infrared laser ablation-based systems for imaging lithographic plates have been developed. These operate by laser-induced ablative removal of organic coating layers, which are coated onto a substrate such as a polyester/metal laminate or onto a polymer coating on a metal support. Use of these polyester or polymer coating materials between the ablation coating and the heat absorbing metal support provides a thermal barrier material which reduces the amount of laser energy required to ablate or fully remove the ablative-absorbing layer and any overlying surface layer, as for example described in Canadian Pat. No. 1,050,805 and in U.S. Pat. Nos. 5,339,737; and 5,353,705. The laser exposure thus removes one or more plate layers, resulting in an imagewise pattern of features on the plate. When the layers removed by laser ablation are the image regions that accept ink, the plates are negative working. When lasers with a large spot size are used for imaging a negative working plate, the size of the smallest printed dot is about as large as the spot size. Consequently, the image quality on printing may not be high. For example, a 35 micron laser spot size would print its smallest dot size at about 35 microns with a negative working plate. On a 200 lines per inch (lpi) halftone screen, this is equivalent to a 5% to 6% dot.
U.S. Pat. No. 5,353,705 discloses a basic plate construction of a lithographic plate having a secondary ablation layer intermediate between a substrate and a surface layer, such as a hydrophilic metal substrate and a radiation-absorptive and ablatively absorbing surface layer. The secondary ablation layer performs the protective or thermal barrier function that shields the substrate from the thermal effects of imaging radiation. The secondary ablation or thermal barrier layer of the ""705 patent is ablated only partially in response to ablation of the ablative-absorbing layer, is preferably substantially transparent to the laser radiation and thereby not characterized by ablative absorption of imaging radiation, and differs from the surface layer in its affinity for at least one printing fluid selected from the group consisting of ink and a fluid that repels ink, i.e., when the surface layer is ink-receptive and/or not receptive to a fountain solution, the thermal barrier layer is not ink-receptive and/or is receptive to a fountain solution, respectively. When the basic plate construction described in the ""705 patent has an ink receptive surface layer, and the thermal barrier or secondary ablation layer is receptive to a fountain solution and thus is not ink receptive, a positive working, wet lithographic plate results since the portions not removed by ablation are the image regions that accept ink. Suitable polymeric materials for the secondary ablation layer of the ""705 patent include, but are not limited to, polymethyl methacrylates, cellulosic ethers and esters, polyesters, and polyurethanes. Hexamethoxymethylmelamine with p-toluenesulfonic acid may be added to these polymeric materials.
U.S. Pat. No. 5,493,971 describes an example of such a positive working, wet lithographic plate. Its plate construction includes a hydrophilic metal substrate, a polymeric, hydrophilic protective or thermal barrier coating which also may serve as an adhesion-promoting primer, and an ink-accepting oleophilic surface layer characterized by ablative absorption of imaging radiation. The imaging laser interacts with the ablatable surface layer, causing ablation thereof. After laser ablation imaging which removes at least the surface layer and also at least some of the hydrophilic protective layer as shown in FIG. 2 of the ""971 patent, the plate is then cleaned with a suitable solvent, e.g., water, to remove portions of the hydrophilic protective layer still remaining in the laser-exposed areas. Since the hydrophilic protective layer is partially ablated in the ""971 patent, but is not characterized by ablative absorption of imaging radiation, this hydrophilic protective layer must not absorb the laser imaging radiation. It is thus similar to the secondary ablation layer of the ""705 patent which is partially ablated and may be substantially transparent to the laser imaging radiation and thus not characterized by ablative absorption of the surface layer. In the ""971 patent, depending on the solubility properties of the residual plug of the partially ablated hydrophilic protective layer in the cleaning solvent, e.g., water, the cleaning step reveals the hydrophilic protective coating at less than its original thickness, or reveals the hydrophilic metal substrate in the areas where the hydrophilic protective coating is entirely removed by the cleaning step. After cleaning, the plate behaves like a conventional positive working wet lithographic metal plate on the printing press.
However, adhesion of the remaining ink-accepting surface coating to the hydrophilic protective layer has proven a difficult problem to overcome. Loss of adhesion can result if the protective hydrophilic thermal barrier layer in the image or printing areas of the plate is damaged or degraded during the laser imaging and cleaning process of the ""971 patent. For example, too much solvent or solubilization action by the cleaning solution or the fountain solution on press may erode the walls of the image areas, eliminating the underlying support provided by the hydrophilic barrier layer around the periphery of the image feature and degrading small image elements. This is particularly problematical when the hydrophilic protective coating layer is partially ablated and probably further removed by the cleaning step and the action of the fountain solution such that the original surface of this protective coating layer is removed. This fully exposes the interface between the ink-accepting layer and the hydrophilic protective coating layer, as well as some of the wall of the hydrophilic protective coating layer at the edge of the image feature, to these wet cleaning and fountain solutions. This may lead to a major loss of image quality. Small dots and type may be removed during the cleaning step or early in the print run. Efforts to improve the adhesion of the laser ablatable surface coating and/or its durability to permit longer printing runs typically leads to a significant increase in the laser energy required to image the plate. International Publication No. WO 99/37481 discloses novel positive working, wet lithographic printing plates and methods for preparing such lithographic printing plates, which overcome this adhesion problem.
U.S. Pat. No. 5,605,780 describes a laser-ablatable lithographic printing plate comprising an anodized aluminum support having thereon an oleophilic image-forming layer comprising an infrared-absorbing agent dispersed in a film-forming cyanoacrylate polymer binder. The hydrophilic protective layer has been eliminated. The ""780 patent describes low required laser energy, good ink receptivity, good adhesion to the support, and good wear characteristics. Print runs of more than 8200 impressions are shown in the examples.
U.S. Pat. No. 5,339,737 and Reissue Pat. No. 35,512 describe a variety of ablation-type lithographic plate configurations for use with laser diode imaging apparatus. These configurations include an ablation layer, which volatilizes into gaseous and particulate debris in response to infrared imaging radiation. As used herein, the term xe2x80x9cablationxe2x80x9d refers to the volatilization of a layer or a material into gaseous and particulate debris in response to imaging radiation, which ablation results in a loss of mass or weight in the layer or material. For example, U.S. Pat. No. 5,493,971 describes a complete or 100% ablative loss of the ablative layer during the laser ablation imaging process, and FIG. 3A of International Publication No. WO 99/37481 describes a partial ablative loss of about 50% or greater of the ablatable layer during the laser ablation imaging process.
Lithographic printing members are now commonly imaged by lower-power laser ablation imaging mechanisms. A major problem with these infrared laser ablation-based systems for imaging lithographic plates has been environmental. Because these operate by laser-induced destruction or removal of organic polymers and other organic or inorganic materials which are coated in one or more layers overlying a substrate, airborne debris and vapors are produced during imaging which may be hazardous to the laser equipment and to the personnel who operate the equipment. Expensive equipment is generally required to contain the debris and to capture the gases.
Despite the many efforts directed to the development of a laser imageable wet lithographic printing plate, there still remains a need for plates that require no alkaline or solvent developing solution, that perform like a conventional lithographic printing plate on press, that are sensitive to a broad spectrum of laser energy such as 700 nm to 1150 nm, that provide a high resolution and durable image, and that do not produce debris and vapor requiring expensive and complex containment equipment.
One aspect of the present invention pertains to methods of imaging a wet lithographic printing member, which methods comprise the steps of (a) providing a positive working lithographic printing member, which positive working member comprises a substrate, a hydrophilic layer overlying the substrate, and an ink-accepting and infrared-absorbing surface layer overlying the hydrophilic layer; wherein the surface layer is characterized by absorption of infrared imaging radiation, by being not removable by cleaning with water or a cleaning solution prior to the absorption of infrared imaging radiation, and by being adapted to form a wet lithographic printing surface as a result of an imagewise exposure to absorbable infrared radiation and subsequent removal of the exposed areas of the surface layer by cleaning with water or the cleaning solution to reveal the underlying hydrophilic layer; and the hydrophilic layer is characterized by being not removable by cleaning with water or the cleaning solution; (b) exposing the positive working member of step (a) to absorbable infrared radiation using an infrared-emitting laser to effect absorption of infrared radiation in the laser-exposed areas of the surface layer that is sufficient to cause the surface layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 10% by weight of the surface layer material in the laser-exposed areas; and (c) removing, with water or the cleaning solution, the laser-exposed areas of the surface layer to reveal the underlying hydrophilic layer. In one embodiment of the methods, the hydrophilic layer is characterized by the absence of removal of the hydrophilic layer in the laser-exposed areas during steps (b) and (c). In a preferred embodiment, the absorption of infrared radiation in the laser-exposed areas of the surface layer of step (b) is sufficient to cause the surface layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 5% by weight, more preferably greater than 2% by weight, and most preferably none, of the surface layer in the laser-exposed areas. Thus, the methods of this invention provide a wet lithographic printing member with a very low or zero level of emission of gases and airborne debris during laser imaging, while also achieving excellent cleanability of the laser-exposed areas by water or an equivalent environmentally-acceptable aqueous solution and excellent image resolution and durability due to the properties of the infrared-absorbing and hydrophilic layers and their interface, as described herein.
Another aspect of the methods of imaging a wet lithographic printing member of this invention pertains to the inclusion of a primer layer interposed between the hydrophilic layer and the infrared-absorbing layer to further enhance the desirable properties of the infrared-absorbing and hydrophilic layers and their interface which greatly influence the amount of airborne materials produced during laser imaging, the speed of laser imaging, the ease of cleanability by water during removal of the laser-exposed areas, and the image resolution and durability, as described herein. In one embodiment of the methods, a primer layer is interposed between the hydrophilic layer and the surface layer in step (a) of the methods of this invention described hereinabove, which primer layer comprises an adhesion-promoting agent. In one embodiment, the thickness of the primer layer of step (a) is from 0.01 to 0.1 microns. In one embodiment, the adhesion-promoting agent of the primer layer comprises a crosslinked, polymeric reaction product of a hydrophilic polymer and a crosslinking agent, and preferably, further comprises a catalyst. In one embodiment, the primer layer comprises an organic sulfonic acid component. In one embodiment, the primer layer comprises a zirconium compound.
The term xe2x80x9cprinting member,xe2x80x9d as used herein, is synonymous with the term xe2x80x9cplatexe2x80x9d and pertains to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or fountain solution. The term xe2x80x9ccleaning solution,xe2x80x9d as used herein, pertains to a solution used to clean or remove the coating or coatings from the laser-exposed regions of the print members of the methods of this invention and may be water, combinations of at least 90% water and 10% or less organic solvents and additives such as alcohols, surfactants, and glycols, and buffered or salt-containing neutral or nearly neutral water solutions, such as known in the art of aqueous fountain solutions for wet lithographic printing. The term xe2x80x9ccleaning solution,xe2x80x9d as used herein, does not include alkaline aqueous solutions with a pH of greater than about 10, acidic aqueous solutions with a pH of less than about 3.5, or organic solvents without at least 90% by weight of water present. In a preferred embodiment, the surface layer in the areas not exposed by the laser is further characterized by being not removable by cleaning with water or a cleaning solution and by durability on a wet lithographic printing press.
In one embodiment, the weight of the infrared-absorbing layer of the printing members of the methods of this invention is from about 0.05 to about 1.0 g/m2. In a preferred embodiment, the weight of the infrared-absorbing layer is from about 0.1 to about 0.5/m2.
In one embodiment of the methods of this invention, the infrared-absorbing layer, which is the surface layer in a two layer plate construction of a hydrophilic layer and an infrared-absorbing layer on a substrate, comprises one or more polymers and an infrared-absorbing sensitizer. In one embodiment, the infrared-absorbing sensitizer is a carbon black. In one embodiment, the infrared-absorbing layer comprises one or more carbon blacks selected from the group consisting of: sulfonated carbon blacks having sulfonated groups on the surface of the carbon black, carboxylated carbon blacks having carboxyl groups on the surface of the carbon black, and carbon blacks having a surface active hydrogen content of not less than 1.5 mmol/g. In a preferred embodiment, the infrared-absorbing sensitizer is CAB-O-JET 200. In another preferred embodiment, the infrared-absorbing sensitizer is BONJET BLACK CW-1. In one embodiment, the infrared-absorbing sensitizer is present in an amount greater than 55% by weight of the infrared-absorbing layer. In one embodiment, the infrared-absorbing sensitizer is present in an amount greater than 65% by weight of the infrared-absorbing layer.
In one embodiment of the methods of this invention, one of the one or more polymers of the infrared-absorbing layer comprises a polymer selected from the group consisting of: polyvinyl alcohols, polyurethanes, epoxy polymers, vinyl polymers, acrylic polymers, and cellulosics. In one embodiment, the infrared-absorbing layer comprises a polyvinyl alcohol. In one embodiment, the polyvinyl alcohol is present in an amount of 20 to 95 per cent by weight of the total weight of polymers present in the infrared-absorbing layer. As used herein, for the purposes of determining the weight per cent of a material, the term xe2x80x9cpolymersxe2x80x9d includes all the materials which are polymeric film formers, including monomeric species which polymerize or combine with a polymeric species, such as, for example, a monomeric crosslinking agent. In one embodiment, the polyvinyl alcohol is present in an amount of 25 to 75 per cent by weight of the total weight of polymers present in the infrared-absorbing layer. Suitable polymers for use in combination with polyvinyl alcohol in the infrared-absorbing layer include, but are not limited to other water-soluble or water-dispersible polymers such as, for example, polyurethanes, cellulosics, epoxy polymers, acrylic polymers, and vinyl polymers.
In one embodiment of the methods of the present invention, the infrared-absorbing layer comprises a crosslinking agent, preferably a melamine. In one embodiment, one or more polymers of the infrared-absorbing layer comprise a crosslinked, polymeric reaction product of a polymer and a crosslinking agent. In a preferred embodiment, the crosslinked, polymeric reaction product is selected from the group consisting of crosslinked reaction products of a crosslinking agent with the following polymers: a polyvinyl alcohol; a polyvinyl alcohol and a vinyl polymer; a cellulosic polymer; a polyurethane; an epoxy polymer; an acrylic polymer; and a vinyl polymer.
In one embodiment of the methods of the present invention, the infrared-absorbing layer further comprises a catalyst in addition to one or more polymers and an infrared-absorbing sensitizer.
In one embodiment of the methods of this invention, the infrared-absorbing layer comprises one or more polymers, an infrared-absorbing sensitizer, and an organic sulfonic acid component, preferably a component of an amine-blocked p-toluenesulfonic acid. In one embodiment, the organic sulfonic acid component is present in an amount of 25 to 75 weight per cent of the total weight of polymers present in the infrared-absorbing layer of the printing members of the methods of the present invention. In another embodiment, the organic sulfonic acid component is present in an amount of 35 to 55 weight per cent of the total weight of polymers present in the infrared-absorbing layer. In one embodiment, the infrared-absorbing layer comprises greater than 5% by weight of the organic sulfonic acid component. In one embodiment, the infrared-absorbing layer comprises greater than 12% by weight of the organic sulfonic acid component.
In one embodiment of the methods of preparing a wet lithographic printing member of the present invention, the hydrophilic layer comprises a crosslinked, polymeric reaction product of a hydrophilic polymer and a first crosslinking agent. Suitable hydrophilic polymers for the crosslinked, polymeric reaction product include, but are not limited to, polyvinyl alcohols and cellulosics. In a preferred embodiment, the hydrophilic polymer is a polyvinyl alcohol. In one embodiment, the first crosslinking agent is a zirconium compound. In one embodiment, the first crosslinking agent is ammonium zirconyl carbonate. In a preferred embodiment, the first crosslinking agent is ammonium zirconyl carbonate, and the ammonium zirconyl carbonate is present in an amount greater than 10% by weight of the polyvinyl alcohol, and, more preferably, present in an amount of 20 to 50% by weight of the polyvinyl alcohol. In another preferred embodiment, the hydrophilic layer further comprises a second crosslinking agent. In one embodiment, the hydrophilic layer further comprises a crosslinked, polymeric reaction product of a polyvinyl alcohol and the second crosslinking agent. In one embodiment, the second crosslinking agent is a melamine. In one embodiment, the hydrophilic layer further comprises a catalyst for the second crosslinking agent. In one embodiment, the catalyst is an organic sulfonic acid component. In one embodiment, the hydrophilic layer comprises an inorganic xerogel layer, which xerogel layer preferably comprises a zirconium oxide xerogel.
In one embodiment of the printing members of the methods of the present invention, the thickness of the hydrophilic layer is from about 1 to about 40 microns. In one embodiment, the thickness of the hydrophilic layer is from about 2 to about 25 microns.
In one embodiment of the printing members of the methods of preparing a wet lithographic printing plate member of this invention, suitable substrates comprise non-metal substrates and non-hydrophilic substrates, preferably papers, polymeric films, and non-hydrophilic metals such as non-hydrophilic aluminum. In one embodiment, the substrate is selected from the group of polymeric films consisting of: polyesters, polycarbonates, and polystyrene. In one embodiment, the polyester polymeric film is a polyethylene terephthalate film. In one embodiment, the non-hydrophilic metal substrate comprises a non-hydrophilic polymer layer on at least one surface of the non-hydrophilic metal substrate. In one embodiment, the substrate is a hydrophilic metal. Suitable metals for the hydrophilic metal substrate include, but are not limited to, aluminum, copper, steel, and chromium. In a preferred embodiment, the metal substrate is grained, anodized, silicated, or a combination thereof. In one embodiment, the metal substrate is aluminum. In a preferred embodiment, the metal substrate is an aluminum substrate comprising a surface of uniform, non-directional roughness and microscopic depressions, which surface is in contact to the hydrophilic layer and, more preferably, this surface of the aluminum substrate has a peak count in the range of 300 to 450 peaks per linear inch which extend above and below a total bandwidth of 20 microinches.
Another aspect of the present invention pertains to methods of preparing a wet lithographic printing member, which methods comprise the steps of (a) coating onto a substrate a liquid mixture comprising a first liquid medium, a hydrophilic polymer, and a first crosslinking agent; (b) drying the layer formed in step (a) to remove the first liquid medium, to cause a portion of the first crosslinking agent present to react, and to form a hydrophilic layer; (c) coating onto the hydrophilic layer a liquid mixture comprising a second liquid medium, a polymer, an infrared-absorbing sensitizer, and a second crosslinking agent; (d) drying the layer formed in step (c) to remove the second liquid medium, to cause an additional portion of the first crosslinking agent present in the hydrophilic layer to react, to cause a portion of the second crosslinking agent present to react, and to form an ink-accepting and infrared-absorbing surface layer; thereby forming a positive working lithographic printing member, wherein the surface layer and the hydrophilic layer are characterized by being not removable by cleaning with water or a cleaning solution; (e) exposing the positive working member of step (d) to absorbable infrared radiation using an infrared-emitting laser to effect absorption of infrared radiation in the laser-exposed areas of the surface layer that is sufficient to cause the surface layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 10% by weight of the surface layer in the laser-exposed areas; and (f) removing, with water or the cleaning solution, the laser-exposed areas of the surface layer to reveal the underlying hydrophilic layer. In one embodiment of the methods, subsequent to step (b) and prior to step (c), there are two steps of (i) coating onto the hydrophilic layer a liquid mixture comprising a liquid medium and an adhesion-promoting agent; and (ii) drying the layer formed in step (i) to remove the liquid medium of step (i) and to form a primer layer; and step (c) then comprises coating onto the primer layer the liquid mixture comprising a second liquid medium, a polymer, an infrared-absorbing sensitizer, and a second crosslinking agent. In one embodiment of the methods, the hydrophilic layer is characterized by the absence of removal of the hydrophilic layer in the laser-exposed areas during steps (e) and (f). In a preferred embodiment, the absorption of infrared radiation in the laser-exposed areas of step (e) is sufficient to cause the surface layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 5% by weight, more preferably greater than 2% by weight, and most preferably none, of the surface layer in the laser-exposed areas. In one embodiment, the weight of the infrared-absorbing layer of the printing members of the methods of this invention is from about 0.05 to about 1.0 g/m2. In a preferred embodiment, the weight of the infrared-absorbing layer is from about 0.1 to about 0.5 g/m2.
Another aspect of the present invention pertains to methods of preparing a wet lithographic printing member, which methods comprise the steps of: (a) coating onto a substrate a liquid mixture comprising a first liquid medium, a hydrophilic polymer, and a first crosslinking agent; (b) drying the layer formed in step (a) to remove the first liquid medium and to form a hydrophilic layer; (c) coating onto the hydrophilic layer a liquid mixture comprising a second liquid medium, a polymer, an infrared-absorbing sensitizer, and a second crosslinking agent; wherein a portion of the second crosslinking agent penetrates into the hydrophilic layer; (d) drying the layer formed in step (c) and the underlying hydrophilic layer to remove the second liquid medium, to cause a portion of the second crosslinking agent present in the hydrophilic layer to react, and to form an ink-accepting and infrared-absorbing surface layer; thereby forming a positive working lithographic printing member, wherein the surface layer and the hydrophilic layer are characterized by being not removable by cleaning with water or a cleaning solution; (e) exposing the positive working member of step (d) to absorbable infrared radiation using an infrared-emitting laser to effect absorption of infrared radiation in the laser-exposed areas of the surface layer that is sufficient to cause the surface layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 10% by weight of the surface layer in the laser-exposed areas; and (f) removing, with water or the cleaning solution, the laser-exposed areas of the surface layer to reveal the underlying hydrophilic layer. In one embodiment of the methods, subsequent to step (b) and prior to step (c), there are two steps of (i) coating onto the hydrophilic layer a liquid mixture comprising a liquid medium and an adhesion-promoting agent; and (ii) drying the layer formed in step (i) to remove the liquid medium of step (i) and to form a primer layer; and step (c) then comprises coating onto the primer layer the liquid mixture comprising a second liquid medium, a polymer, an infrared-absorbing sensitizer, and a second crosslinking agent. In one embodiment of the methods, the hydrophilic layer is characterized by the absence of removal of the hydrophilic layer in the laser-exposed areas during steps (e) and (f). In a preferred embodiment, the absorption of infrared radiation in the laser-exposed areas of the surface layer of step (e) is sufficient to cause the surface layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 5% by weight, more preferably greater than 2% by weight, and most preferably none, of the surface layer in the laser-exposed areas. In one embodiment, the weight of the infrared-absorbing layer is from about 0.05 to about 1.0 g/m2. In a preferred embodiment, the weight of the infrared-absorbing layer is from about 0.1 to about 0.5 g/m2.
Still another aspect of this invention pertains to methods of preparing a wet lithographic printing member, which methods comprise the steps of: (a) coating onto a substrate a liquid mixture comprising a first liquid medium, one or more hydrophilic polymers, and a first crosslinking agent, wherein the first crosslinking agent is present in an amount greater than 10% by weight of the one or more hydrophilic polymers; (b) drying the layer formed in step (a) to remove the first liquid medium and to form a hydrophilic layer; (c) coating onto the hydrophilic layer a liquid mixture comprising a second liquid medium, a polymer, an infrared-absorbing sensitizer, and a second crosslinking agent; (d) drying the layer formed in step (c) to remove the second liquid medium and to form an ink-accepting and infrared-absorbing surface layer, wherein the sensitizer is present in an amount of 25 to 80% by weight of the surface layer, and the one or more polymers are present in an amount of 10 to 60% by weight of the surface layer; thereby forming a positive working lithographic printing member, wherein the surface layer and the hydrophilic layer are characterized by being not removable by cleaning with water or a cleaning solution; (e) exposing the positive working member of step (d) to absorbable infrared radiation using an infrared-emitting laser to effect absorption of infrared radiation in the laser-exposed areas of the surface layer that is sufficient to cause the surface layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 10% by weight of the surface layer in the laser-exposed areas; and (f) removing, with water or the cleaning solution, the laser-exposed areas of the surface layer to reveal the underlying hydrophilic layer. In one embodiment of the methods, the hydrophilic layer is characterized by the absence of removal of the hydrophilic layer in the laser-exposed areas during steps (e) and (f). In a preferred embodiment, the absorption of infrared radiation in the laser-exposed areas of the surface layer of step (b) is sufficient to cause the surface layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 5% by weight, more preferably greater than 2% by weight, and most preferably none, of the surface layer in the laser-exposed areas. In one embodiment, the weight of the infrared-absorbing surface layer is from about 0.05 to about 1.0 g/m2. In one embodiment, the weight of the infrared-absorbing surface layer is from about 0.1 to about 0.5 g/m2. Suitable polymers for forming the ink-accepting and infrared-absorbing layer of steps (c) and (d) include, but are not limited to, polyvinyl alcohols, polyurethanes, epoxy polymers, vinyl polymers, acrylic polymers, and cellulosics.
Another aspect of the methods of imaging a wet lithographic printing member of the present invention pertains to the inclusion of an additional ink-accepting surface layer overlying the infrared-absorbing layer to provide a basic three layer product design of ink-accepting surface layer/infrared-absorbing layer/hydrophilic layer on the substrate. This additional ink-accepting surface layer may be useful in achieving the best overall balance of properties, such as increasing the speed of laser imaging, the ease of cleanability by water during removal of the laser-exposed areas, and particularly the image resolution and durability. In one embodiment, the method of imaging a wet lithographic printing member comprises the steps of (a) providing a positive working lithographic printing member, which positive working member comprises a substrate, a hydrophilic layer overlying the substrate, an infrared-absorbing layer overlying the hydrophilic layer, and an ink-accepting surface layer overlying the infrared-absorbing layer; the surface layer being characterized by the absence of ablation from absorption of infrared imaging radiation; the infrared-absorbing layer being characterized by absorption of imaging radiation; the surface layer and the infrared-absorbing layer being characterized by being not removable by cleaning with water or a cleaning solution prior to the absorption of infrared imaging radiation and by being adapted to form a wet lithographic printing surface as a result of an imagewise exposure to absorbable infrared radiation and subsequent removal of the exposed areas of the surface layer and the infrared-absorbing layer by cleaning with water or the cleaning solution to reveal the underlying hydrophilic layer; and the hydrophilic layer being characterized by being not removable by cleaning with water or the cleaning solution; (b) exposing the positive working member of step (a) to absorbable infrared radiation using an infrared-emitting laser to effect absorption of infrared radiation in the laser-exposed areas of the infrared-absorbing layer that is sufficient to cause the surface layer and the infrared-absorbing layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 10% by weight of the combined surface layer and infrared-absorbing layer in the laser-exposed areas; and (c) removing, with water or the cleaning solution, the laser-exposed areas of the surface layer and the infrared-absorbing layer to reveal the underlying hydrophilic layer.
Another aspect of the methods of imaging a wet lithographic printing member with a basic three layer product design of this invention pertains to the inclusion of a primer layer interposed between the hydrophilic layer and the infrared-absorbing layer to further enhance the desirable properties of the infrared-absorbing and hydrophilic layers and their interface which greatly influence the amount of airborne materials produced during laser imaging, the speed of laser imaging, the ease of cleanability by water during removal of the laser-exposed areas, and the image resolution and durability, as described herein. In one embodiment of the methods, a primer layer is interposed between the hydrophilic layer and the infrared-absorbing layer in step (a) of the methods, which primer layer comprises an adhesion-promoting agent. In one embodiment, the thickness of the primer layer of step (a) is from 0.01 to 0.1 microns. In one embodiment, the adhesion-promoting agent of the primer layer comprises a crosslinked, polymeric reaction product of a hydrophilic polymer and a crosslinking agent, and preferably, further comprises a catalyst. In one embodiment, the primer layer comprises an organic sulfonic acid component. In one embodiment, the primer layer comprises a zirconium compound.
In one embodiment of the methods of imaging a wet lithographic printing member with a three-layer product design with optional primer layer of this invention, the ink-accepting surface layer comprises a crosslinked, polymeric reaction product of a polymer and a crosslinking agent. Suitable polymers for the crosslinked, polymeric reaction product includes, but are not limited to, cellulosics, acrylic polymers, polyurethanes, and epoxy polymers. In one embodiment, the ink-accepting surface layer further comprises an organic sulfonic acid component. In one embodiment, the weight of the ink-accepting surface layer is from about 0.05 to about 0.5 g/m2. In a preferred embodiment, the weight of the ink-accepting surface layer is from about 0.1 to about 0.3 g/m2. In one embodiment of the methods, the hydrophilic layer is characterized by the absence of removal of the hydrophilic layer in the laser-exposed areas during steps (b) and (c). In a preferred embodiment, the absorption of infrared radiation in the laser-exposed areas of the infrared-absorbing layer of step (b) is sufficient to cause the ink-accepting surface layer and the infrared-absorbing layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 5% by weight, more preferably greater than 2% by weight, and most preferably none, of the combined ink-accepting surface layer and infrared-absorbing layer in the laser-exposed areas.
Another aspect of the present invention pertains to methods of preparing a wet lithographic printing member with a basic three layer product design, which methods comprise the steps of (a) coating onto a substrate a liquid mixture comprising a first liquid medium, a hydrophilic polymer, and a first crosslinking agent; (b) drying the layer formed in step (a) to remove the first liquid medium, to cause a portion of the first crosslinking agent present to react, and to form a hydrophilic layer; (c) coating onto the hydrophilic layer a liquid mixture comprising a second liquid medium, a polymer, an infrared-absorbing sensitizer, and a second crosslinking agent; (d) drying the layer formed in step (c) to remove the second liquid medium, to cause an additional portion of the first crosslinking agent present in the hydrophilic layer to react, to cause a portion of the second crosslinking agent present to react, and to form an infrared-absorbing layer; (e) coating onto the infrared-absorbing layer a liquid mixture comprising a third liquid medium and an ink-accepting polymer; (f) drying the layer formed in step (e) to remove the third liquid medium and to form an ink-accepting surface layer; thereby forming a positive working wet lithographic printing member, wherein the surface layer, the infrared-absorbing layer, and the hydrophilic layer are characterized by being not removable by cleaning with water or a cleaning solution; (g) exposing the positive working member of step (f) to absorbable infrared radiation using an infrared-emitting laser to effect absorption of infrared radiation in the laser-exposed areas of the infrared-absorbing layer that is sufficient to cause the surface layer and the infrared-absorbing layer in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 10% by weight of the combined surface layer and infrared-absorbing layer in the laser-exposed areas; and (h) removing, with water or the cleaning solution, the laser-exposed areas of the surface and infrared-absorbing layers to reveal the underlying hydrophilic layer. In one embodiment of the methods, subsequent to step (b) and prior to step (c), there are two steps of (i) coating onto the hydrophilic layer a liquid mixture comprising a liquid medium and an adhesion-promoting agent; and (ii) drying the layer formed in step (i) to remove the liquid medium of step (i) and to form a primer layer; and step (c) then comprises coating onto the primer layer the liquid mixture comprising a second liquid medium, a polymer, an infrared-absorbing sensitizer, and a second crosslinking agent.
Still another aspect of the methods of preparing a wet lithographic printing member having a three layer product design of this invention pertains to methods comprising the steps of (a) coating onto a substrate a liquid mixture comprising a first liquid medium, a hydrophilic polymer, and a first crosslinking agent; (b) drying the layer formed in step (a) to remove the first liquid medium and to form a hydrophilic layer; (c) coating onto the hydrophilic layer a liquid mixture comprising a second liquid medium, a polymer, an infrared-absorbing sensitizer, and a second crosslinking agent; wherein a portion of the second crosslinking agent penetrates into the hydrophilic layer; (d) drying the layer formed in step (c) and the underlying hydrophilic layer to remove the second liquid medium, to cause a portion of the second crosslinking agent present in the hydrophilic layer to react, and to form an infrared-absorbing layer; (e) coating onto the infrared-absorbing layer a liquid mixture comprising a third liquid medium and an ink-accepting polymer; (f) drying the layer formed in step (e) to remove the third liquid medium and to form an ink-accepting surface layer; thereby forming a positive working wet lithographic printing member, wherein the surface layer, the infrared-absorbing layer, and the hydrophilic layer are characterized by being not removable by cleaning with water or a cleaning solution; (g) exposing the positive working member of step (f) to absorbable infrared radiation using an infrared-emitting laser to effect absorption of infrared radiation in the laser-exposed areas of the infrared-absorbing layer that is sufficient to cause the surface and infrared-absorbing layers in the laser-exposed areas to become removable by cleaning with water or the cleaning solution but insufficient to remove by ablation greater than 10% by weight of the combined surface layer and infrared-absorbing layer in the laser-exposed areas; and (h) removing, with water or the cleaning solution, the laser-exposed areas of the surface and infrared-absorbing layers to reveal the underlying hydrophilic layer. In one embodiment of the methods, subsequent to step (b) and prior to step (c), there are two steps of (i) coating onto the hydrophilic layer a liquid mixture comprising a liquid medium and an adhesion-promoting agent; and (ii) drying the layer formed in step (i) to remove the liquid medium of step (i) and to form a primer layer; and step (c) then comprises coating onto the primer layer the liquid mixture comprising a second liquid medium, a polymer, an infrared-absorbing sensitizer, and a second crosslinking agent.
In one embodiment, the infrared-absorbing layer of the three layer designs of the printing members of the methods of the present invention is ink-accepting. In one embodiment, the infrared-absorbing layer of the three layer designs of the printing members of the methods of the present invention is further characterized by not accepting ink and by accepting water on a wet lithographic printing press.
Another aspect of this invention pertains to methods for preparing a positive working, wet lithographic printing member, for both two layer and three layer product designs with highly crosslinked layers and with various approaches for interaction of the crosslinking chemistry by interfacial reactions between adjacent infrared-absorbing and hydrophilic layers. The infrared-absorbing sensitizers in the infrared-absorbing layer for use with the highly crosslinked layers of the present invention are not limited to organic sensitizers, such as carbon blacks and organic dyes, but may include inorganic and metallic sensitizers.
Still another aspect of the present invention pertains to wet lithographic printing members prepared according to the methods of this invention.
One advantage of the present invention is that the lithographic printing member or plate may be imaged at very low laser power, which eliminates ablation of the infrared-absorbing layer and of the ink accepting surface layer, if present, thus eliminating almost all or all noxious vapors and airborne debris. Since the water-based fountain solution on the wet lithographic printing press will easily clean the laser-exposed infrared-absorbing layer, and also the ink-accepting surface layer if present, from the plate, the plate is suitable for on press imaging and direct printing. Also, in the course of a long printing run, the hydrophilic layer is not solubilized by the fountain solution, and non-hydrophilic substrates may be utilized. Further, the hydrophilic layer under the non-exposed image areas of the present invention provides excellent adhesion to the overlying ink-accepting image layer since it is nearly impossible to undercut through solubilization, particularly when the hydrophilic layer is highly crosslinked, including at its interface to the infrared-absorbing layer.
The superiority of the methods and of the lithographic printing members of the methods of the present invention over those previously known is particularly manifest in the ability to be imaged rapidly with relatively inexpensive diode lasers with large spot sizes; its low laser power imaging characteristic; its elimination of noxious vapors and airborne debris during imaging; its ease of cleaning; its excellent image resolution and printing quality; its resistance to water, which provides excellent durability and image adhesion on the printing press; and its low cost of manufacture.
As one of skill in the art will appreciate, features of one embodiment and aspect of the invention are applicable to other embodiments and aspects of the invention The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.